The present invention relates to a process for producing scratch-resistant coatings, especially scratch-resistant multicoat finishes.
The present invention relates, furthermore, to coating compositions suitable for this process.
In past years, great progress has been made in developing acid-resistant and etch-resistant clearcoats for the OEM finishing of automobiles. More recently, there is now an increasing desire in the automotive industry for scratch-resistant clearcoats which at the same time retain the level attained hitherto in terms of their other properties.
At present, however, there are different test methods for the quantitative assessment of the scratch resistance of a coating, examples being testing by means of the BASF brush test, by means of the washing brush unit from the company AMTEC, or various test methods of automakers and others. A disadvantage, however, is that it is frequently impossible to correlate the individual test results; In other words, the test results for one and the same coating may have very different outcomes depending on the test method chosen, and passing one scratch resistance test does not, under certain circumstances, permit conclusions to be drawn about the behavior of that coating in a different scratch test.
There is therefore a desire for a standardized method of quantitatively assessing the scratch resistance which enables reliable statements to be made about the scratch resistance of the coating from just one examination of the sample. In particular, the result of this examination should permit reliable conclusions to be drawn about the scratch resistance of the coating in the various abovementioned scratch resistance tests.
The literature, indeed, has already described a number of investigations relating to the physical processes taking place during the production of scratches in a coating, and correlations, derived therefrom, between the scratch resistance and other physical parameters of the coating. A contemporary review of various literature relating to scratch-resistant coatings can be found, for example, in J. L. Courter, 23rd Annual International Waterborne, High-Solids and Powder Coatings Symposium, New Orleans 1996.
Furthermore, for example, the article by S. Sano et al., xe2x80x9cRelationship between Viscoelastic Property and Scratch Resistance of Top-Coat Clear Filmxe2x80x9d, Toso Kagaku 1994, 29 (12), pages 475-480 uses a washing brush test to determine the scratch resistance of different, heat-curing melamine/acrylate or isocyanate/acrylate systems and correlates the scratch resistance found with viscoelastic properties of the coating.
From the test results described in that article, the authors conclude that coatings would exhibit good scratch resistance when either the so-called xe2x80x9cinter-crosslinking molecular weightxe2x80x9d was below 500 or when the glass transition temperature was 15xc2x0 C. or less, it being necessary, however, in the case of clearcoat films in the automotive sector, for the glass transition temperature to be above 15xc2x0 C. in order to achieve sufficient hardness of the coatings.
In the article by M. Rxc3x6sler, E. Klinke and G. Kunz in Farbe+Lack, Volume 10, 1994, pages 837-843, too, the scratch resistance of various coatings is investigated by means of different test methods. The article found that, under a given load, hard coatings exhibit greater damage and thus lower scratch resistance than soft coatings.
Still further, in the conference report of B. V. Gregorovich and P. J. McGonical, Proceedings of the Advanced Coatings Technology Conference, Illinois, USA, Nov. 3-5, 1992, pages 121-125, it is found that increasing the plastic nature (toughness) of coatings improves the scratch resistance, owing to the improvement in plastic flow (greater healing), although limits are imposed on the increase in plastic nature by the other properties of the coating.
Further yet, P. Betz and A. Bartelt in Progress in Organic Coatings, 22 (1993), pages 27-37 disclose various methods of determining the scratch resistance of coatings. That article makes reference, furthermore, to the fact that the scratch resistance of coatings is influenced not only by the glass transition temperature but also, for example, by the homogeneity of the network.
That article proposes increasing the scratch resistance of clearcoat coatings by incorporating siloxane macromonomers, since these siloxane macromonomers lead to increased homogeneity of the clearcoat surface and, above 60xc2x0 C., to an improved plastic flow.
The correlation between storage modulus and crosslinking density, furthermore, is known, for example, from Loren W. Hill, Journal of Coatings Technology, Vol. 64, No. 808, May 1992, pages 29 to 41. However, that article contains no statements or indications as to how scratch-resistant coatings can be obtained.
DE-C-39 18 968, furthermore, discloses a process for coating surfaces using clearcoats, based on hydroxyl-containing resins and polyisocyanates, whose composition is established such that the clearcoat film, after curing, has a molecular weight of the chain between the crosslinks of up to 200 (measured in accordance with the xylene swelling method). However, even these clearcoats are still in need of improvement in respect of the scratch resistance of the resultant coatings.
Finally, DE-A-43 10 414 and DE-A-42 04 518 disclose nonaqueous clearcoats based on hydroxyl-containing acrylate resins and isocyanates, for the production of multicoat finishes, where the resulting coatings are notable for improved scratch resistance and good other service properties. However, even with these clearcoats there is a desire for an even greater improvement in scratch resistance.
Although many scratch-resistant finishes and method of producing same are known, a need still exists in the art for a process for producing coatings having further-improved scratch resistance. At the same time, the coating compositions employed in this process should, furthermore, be suitable as a clearcoat and/or topcoat for producing a multicoat finish, especially in the automotive sector. In addition, the coatings should exhibit high gloss, good chemical resistance and good weathering stability.
The object of the present invention is, therefore, to provide such coating compositions and a process for producing coating using such coating compositions which fulfills the need in the art.
It is another object of the present invention to establish a criterion for assessing the scratch resistance of the cured coating objectively, independently of the particular test method chosen, on the basis of physical parameters. The method of determining the physical parameters according to the invention is able to be used under practical conditions and with sufficient accuracy to enable the scratch resistance to be characterized in a way which is at least adequate to visual evaluation.
These object are, surprisingly, achieved by a process for producing scratch-resistant coatings which comprises the steps of providing at least one coating composition and forming a scratch-resistant coating on a surface using the coating compositions wherein the coating compositions,
after curing, have a storage modulus Exe2x80x2 in a rubber-elastic range of at least 107.5 Pa and a loss factor tan xcex4 at 20xc2x0 C. of at least 0.05, the storage modulus Exe2x80x2 and the loss factor tan xcex4 having been measured by dynamic mechanical thermoanalysis on homogeneous free films having a film thickness of 40xc2x110 xcexcm the coating compositions comprise
as binders one or more polyacrylate resins having a hydroxyl number of from 100 to 240, preferably more than 160 to 220 and, with particular preference, from 170 to 200, an acid number of from 0 to 35, preferably from 0 to 25, and a number-average molecular weight of from 1500 to 10,000, preferably from 2500 to 5000, and as crosslinkers one or more free isocyanates, blocked isocyanates and triazine-based components which crosslink with hydroxyl groups of the binders to form ether and/or ester structures.
The present specification relates, furthermore, to a process for producing a scratch-resistant multicoat finish and to coating compositions suitable for this process.
It is surprising and was not foreseeable that, merely by measuring the viscoelastic properties of free films by means of dynamic mechanical thermoanalysis (also referred to for short below as DMTA) there is available a universal, representative selection criterion for the provision of coating compositions which lead to scratch-resistant coatings. At the same time, the results of the DMTA measurements can be correlated with the results of the different test methods for scratch resistance, so that, on the basis solely of the results of the DMTA measurements, statements are possible about the results in other scratch resistance tests, such as, for example, the BASF brush test or the AMTEC test, or various test methods of the automakers.
Other objects, advantages and salient features of the invention will be apparent from the following detailed description.